NCERT Solutions For Class 7 Science Chapter 6: Physical And Chemical Changes

Solving NCERT questions is a crucial part of a Class 7 student’s final exam preparation as many of the exam questions follow a very similar pattern. This is why Extramarks offers NCERT Solutions for Class 7 Science Chapter 6 so that if students ever face any problems with any of the NCERT questions, they can always refer to these solutions for help. These solutions are prepared by subject matter experts that give special attention to providing accurate solutions in a step-by-step and easy-to-understand manner. Students will find these solutions very helpful with their preparations, and last-minute revisions, and even help them with their assignments.

NCERT Solutions for Class 7 Science Chapter 6 

Access NCERT Solutions for Class 7 Science Chapter 6 – Physical and Chemical Changes

Chapter 6 of Class 7 Science textbook focuses on chemical and physical changes. We are always  surrounded by these changes happening around us all the time. It’s just that we don’t realise that they are there. Some of the examples of chemical and physical changes happening around us include the setting of curd, sugar dissolving in water, etc. This chapter is all about these chemical and physical changes, their properties, and so on.  Students will study many chemical and physical changes in depth in this chapter.

6.1 Physical Changes

A substance’s form, size, colour, and condition are known as its physical properties. When the physical properties of a substance change, it is said to have experienced a physical change. It’s worth noting that most physical changes can get rectified. A physical change does not give rise to the development of a new substance.

6.2 Chemical Changes

You have seen a coating of brownish material on any iron substance. Rust is the substance while rusting is the process. A chemical process causes iron to rust. 

A chemical transformation leads to the production of one or more new compounds. A chemical reaction is a term that refers to a chemical change. During the rusting of iron, we discover a new substance called rust. A chemical change can also be seen in the following examples:

  1. Fruit maturation
  2. Fermentation of Grapes 
  3. The process of food digestion
  4. Burning of coal, wood, or leaves. In reality, burning is usually accompanied by the generation of heat and other gases, burning is a chemical transformation.

We can make gas, we can produce a sound, new material may get synthesised, or we can produce heat during a chemical reaction. A chemical change can also be seen when there is a  change in smell.

6.3 Rusting of Iron

Iron rusting is a chemical reaction. It not only degrades the quality but also eventually destroys iron items. This section will teach students all they need to know about iron rusting, including its characteristics and causes. Take notice of the rusting word equation below. 

Iron + Oxygen + water → Rust

The presence of oxygen and water causes the process of rusting. Iron can get maintained from rusting by not exposing it to oxygen, water, or both. Another way to keep iron from rusting is to apply a metal coating like chromium or zinc. The process of galvanising involves coating iron with zinc.

6.4 Crystallisation

Crystallisation is a physical transformation. It is the process of forming large crystals of a pure substance. Physical and chemical changes are now well-known to you. You can create a list of the changes you see in your environment and try to categorise them as physical or chemical changes. That little practice will aid your comprehension of the idea.

Key Features of NCERT Solutions for Class 7 Science Chapter 6

Chapter 6 of Class 7 Science introduces some quite technical terms and concepts to students related to physical and chemical changes. To ensure that students are able to understand the solutions prepared by Extramarks, they are prepared in an easy-to-understand language, explaining the technical jargon wherever possible.

Some of the key features of NCERT Solutions for Class 7 Science Chapter 6 are listed below.

  • The solutions are prepared in easy-to-understand language and include examples and equations wherever necessary. .
  • Students can utilise these resources to effectively manage their exam preparations. They can also get an idea as to how they should handle those tricky questions  in final exams.

Q.1 Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
(b) Dissolving sugar in water
(c) Burning of coal
(d) Melting of wax
(e) Beating aluminium to make aluminiun foil
(f) Digestion of food

Ans-

(a) Photosynthesis: Chemical change
(b) Dissolving sugar in water: Physical change
(c) Burning of coal: Chemical change
(d) Melting of wax: Physical change
(e) Beating aluminium to make aluminium foil: Physical change
(f) Digestion of food: Chemical change

Q.2 State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.
(a) Cutting a log of wood into pieces is a chemical change. (True/False)
(b) Formation of manure from leaves is a physical change. (True/False)
(c) Iron pipes coated with zinc do not get rusted easily. (True/False)
(d) Iron and rust are the same substances. (True/False)
(e) Condensation of steam is not a chemical change. (True/False)

Ans-

(a) Cutting a log of wood into pieces is a chemical change. (False)
Corrected statement: Cutting a log of wood into pieces is a physical change.

(b) Formation of manure from leaves is a physical change. (False)
Corrected statement: Formation of manure from leaves is a chemical change.

(c) Iron pipes coated with zinc do not get rusted easily. (True)

(d) Iron and rust are the same substances. (False)
Corrected statement: Iron and rust are different substances.

(e) Condensation of steam is not a chemical change. (True)

Q.3 Fill in the blanks in the following statements:
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of ________.
(b) The chemical name of baking soda is ________.
(c) Two methods by which rusting of iron can be prevented are _________ and _________.
(d) Changes in which only ________ properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called _________ changes.

Ans-

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate.
(b) The chemical name of baking soda is sodium hydrogencarbonate.
(c) Two methods by which rusting of iron can be prevented are painting and galvanisation.
(d) Changes in which only physical properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called chemical changes.

Q.4 When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain

Ans-

When baking soda (sodium hydrogencarbonate) is mixed with lemon juice (citric acid), bubbles are produced due to the formation of carbon dioxide and other substances. It is a chemical change. Therefore, we cannot get back the original substances, i.e., lemon juice and baking soda.

Q.5 When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.

Ans-

When a candle burns, both physical and chemical changes take place.When we burn a candle, the wax melts which can be converted back into solid wax on cooling. This shows that melting of wax is a physical change. Burning of a candle also produces light along with some gases like carbon dioxide. Hence, burning of the wick of a candle is a chemical change.
Cooking food is another example in which both physical and chemical changes take place.
Physical change: appearance of ingredients
Chemical change: new substances are formed

Q.6 How would you show that setting of curd is a chemical change?

Ans-

Setting of curd is type of chemical change as new substance is formed which has different properties from milk. Once curd is formed, milk cannot be re-obtained from it.

Q.7 Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Ans-

Cutting of wood into small pieces is a physical change because no new substance is formed and only the shape and size of wood changes.When we burn wood, carbon dioxide along with some other new substances is formed. Therefore, burning of wood is a type of chemical change.

Q.8 Describe how crystals of copper sulphate are prepared.

Ans-

Crystal of copper sulphate is obtained by the process of crystallisation. For the preparation of crystals of copper sulphate take some water in a beaker and add a few drops of dilute sulphuric acid. Heat the water. When it starts boiling add copper sulphate powder slowly while stirring continuously. Continue adding copper sulphate powder till it stops dissolving. Filter the solution into a china dish and allow it to cool. Do not disturb the solution while it cools. Slowly, the crystals of copper sulphate separate out.

Q.9 Explain how painting of an iron gate prevents it from rusting.

Ans-

The presence of both oxygen (air) and water (moisture) is essential for rusting to take place.When an iron gate is painted, oxygen and water do not come in contact with iron. Hence, rusting is prevented.

Q.10 Explain why rusting of iron object is faster in coastal areas than in deserts.

Ans-

The presence of both oxygen and water is essential for rusting. Greater humidity indicates presence of more amount of water vapours in air, therefore, faster rate of rusting. In coastal areas humidity is high as compared to deserts. Moreover, the presence of salty water also speeds up the process of rusting in coastal areas. Thus, the iron objects rust faster in coastal areas.

Q.11 The gas we use in the kitchen is called liquefied petroleum gas (LPG). In the cylinder it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.

Ans-

The correct option is (ii). Process – B is a chemical change.

Q.12 Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.

Ans-

The correct option is (iii). Both processes A and B are chemical changes.

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FAQs (Frequently Asked Questions)

Any change in a substance’s physical attributes, such as form, size, colour, etc., is known as physical change. It is usually a reversible transformation that does not result in the formation of new material whereas, a chemical change happens when a material mixes with another ingredient to generate a new compound or when a substance experiences a chemical reaction. In most cases, the transformation is irreversible.

Chapter 6 has 12 questions, three of which are short-form while the remaining nine are long-form.

Physical and chemical transformations are two main topics taught in NCERT Class 7 Science Chapter 6. This chapter discusses the differences between physical and chemical changes and the consequences  of these changes.